The present invention relates generally to methods and apparatus for detecting faults on electrical transmission and distribution lines, and more particularly to a system for detecting high impedance faults.
A high impedance fault (HIF) may be defined as a fault that occurs when a downed or otherwise faulted conductor fails to draw enough current to operate a conventional protective relay. Conventional relays must be set above the maximum load current and above any cold load pickup condition that might occur. This degrades the reliability since faults near or below these values cannot be detected. However, since a down conductor on a distribution feeder is primarily a safety issue, an approach that is different from that used for overcurrent protection is required.
Implementation of a HIF detector for feeder tripping may increase outages and create new hazards (e.g., de-energizing traffic signals). Detecting these faults is very difficult since various loads and normal distribution activities (such as capacitor switching) may generate transients that must be distinguished from a fault condition. Detection is further complicated since HIFs are sporadic in nature. For example, when a broken conductor falls on the ground, it may intermittently conduct current. In some cases, the HIF may draw very little if any current. Despite these difficulties, since public safety is a top priority for utilities, there has been a long felt need to solve this problem. Recently, General Electric, Nordon Technologies, and The Kearney Company have developed products to detect HIFs on a feeder.
The GE Digital Fault Monitor (DFM) attempts to distinguish a fault from normal activities. The frequencies and energy present in the signal are inputs to an intelligent system that determines the presence of arcing on the feeder. In addition, RMS and 60 Hz analyses are used to determine the presence of a HIF. The multiple requirements of this scheme are deemed necessary, since some utilities have indicated that a false trip is less desirable than an undetected HIF.
The High Impedance Fault Alarm System (HIFAS) is a relay developed by Nordon Technologies and is commercially available. Rather than examine various frequencies caused by a HIF, the HIFAS relay employs characteristics of the third harmonic. Basically, Nordon Technologies asserts that the third harmonic due to a HIF is 180 degrees out of phase with the fundamental voltage at the fault location. By monitoring the third harmonic and determining the change in the third harmonic from steady state values, the HIFAS identifies a high impedance fault.
Another approach to HIF detection has been proposed by The Kearney Company. Rather than analyze the signal distortions caused by the HIF, the loss of potential is used. The Open Conductor Detector System (OCDS) utilizes a transmitter on the 120 VAC side of a distribution transformer. Loss of voltage on the line causes the transmitter to emit a specified frequency on the system neutral. Reception of the signal at the substation indicates a broken conductor. Since the transducer may act following a blown fuse on the distribution transformer, the use of two transmitters is recommended for redundancy, which may be too expensive for many applications. Thus, an open conductor is only determined if the receiver acknowledges two frequencies (one distinct frequency per transmitter).
Shortcomings of the above-described approaches include: With respect to the Nordon approach, noisy loads may interfere with the operation of the system. The Kearny approach would place radio transmitters at the fuses and houses at which a loss of potential is to be detected. This is believed to be prohibitively expensive. Finally, the GE approach is not deemed to be sufficiently reliable.
A primary goal of the present invention is to provide an improved HIF detection system. Such a system in accordance with the present invention comprises the following elements:
an analyzer located at a circuit breaker or substation with feeders, wherein the analyzer includes means for analyzing current and/or voltage waveforms to detect a HIF on the feeder or on one of a plurality of laterals coupled to the feeder;
a plurality of remote outage detectors located respectively at corresponding customer sites, each remote outage detector including a mechanism to detect a loss of potential or power at the corresponding site; and
a computer in communication with the analyzer and the remote outage detectors.
Other features of the present invention are disclosed below.